The requirement that grease compositions provide adequate lubrication at high temperature for extended periods of time has become increasingly important. For this reason, many grease compositions contain a variety of organic thickening agents, such as those containing multiple uriedo or urea functional groups. A number of patents, e.g., U.S. Pat. Nos. 3,846,314; 3,242,210 and 3,243,372 disclose specialized thickening agents for various fluids, the thickening agents being obtained by reacting a three component reactant mixture comprising a monoamine, a polyamine and a diisocyanate, or a monoisocyanate, a diisocyanate and a polyamine. As a general rule, the reaction product comprises a mixture of urea-containing species of varying chain length and urea content. However, by careful control of reaction variables such as, e.g., the relative quantities of reactants employed, the reaction temperature and the rate and order of reactant mixing, a product may normally be obtained which predominates in one polyurea species. The polyurea reaction is preferably carried out in situ in the grease carrier, and the reaction product may be utilized directly as a grease thickener.
While greases thickened with polyurea thickeners are in many respects superior to older lubricants in severe service application, especially with regard to maintainance of grease consistency at high temperatures, such greases suffer several disadvantages which limit their usefulness under practical service conditions. For example, in some instances, the product must be subjected to rotor/stator shear or high pressure Manton Gaulin milling to get the best penetration yield for the amount of gellant used. Again, while polyurea thickened greases show excellent retention of mechanical properties at high temperature (70.degree. C. or above) and high or low shear, they tend to soften considerably when subjected to low shear at ambient temperature ranges (20.degree.-30.degree. C.). In fact, the tendency to soften at ambient temperature under low shear can be so great that the grease can, when subject to mechanical working under these conditions, undergo a change in penetration grade, e.g., from a No. 2 NLGI penetration grade to a No. 1 NLGI penetration grade. This change in penetration grade at ambient temperature under low shear is particularly troublesome since it may occur under practical use conditions when the grease is transferred from the original shipping container or is otherwise stirred or handled. Consequently, normal handling of the grease in making it available to the ultimate consumer may change it consistency to such extent that it is no longer the desired penetration grade for the intended application. While it is true that the change in consistency is reversible, in that the softened grease can be subjected to high shear at high temperatures (conditions used in the original grease preparation) to return the grease to its original consistency, this reversal often requires that the softened grease be shipped back to the formulator for reprocessing.
Accordingly, a need has existed for the development of a polyurea grease formulation which possesses improved mechanical properties. The invention satisfies that need, and provides improved composite polyurea grease compositions having enhanced mechanical stability.